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Old 3rd March 2010, 03:48 PM   #111
JimT is offline JimT  Canada
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As some others have reported the EMU 0202, other USB audio devices, generate a lot of noise at their output, far above the advertised level. With the high level of noise I was unable to use this for distortion measurements. I decided to investigate on the off chance I could fix it. I found two individual sources of noise, plus some general background noise.


Please note that the numbers in the following are from memory only and are probably inaccurate.


I connected the 0202 up to an HP 332A distortion meter. played a 1KHz tone using the 0202, and got about 3% THD + noise, worse than my Sansa Clip MP3 player. The output of the HP 332A was connected to an oscilloscope. The oscilloscope showed a large repeating “bump” in the noise about every 250ms. RIAA bounces during its measurement and I assume these noise “bumps” are the cause. Searching the internet lead me to check for latency problems, apparently common with both USB and Firewire devices on Windows. This latency is the delay that Windows can take before responding to a request to service the USB, not a frequency latency as we normally have in audio with crossovers. It seems that if the delay gets too long, the 0202 suffers from Windows transfer "glitches" and noise bumps are the result. The latency was measured with the Thesycon’s DPC Latency Checker (DPC Latency Checker). My laptop had about 500us and was supposedly fine for streaming audio and video. As it was snowing outside and these noise bumps could be caused by USB latency problems, I followed their instructions to reduce this latency and was able to get this down to 6us. Rechecking the output on the scope showed the noise bumps were gone and it seems likely they were caused by these USB latency problems. RIAA now measured quite well at a usable low THD+noise of about -110dB and bouncing during the test was fixed. It seems Windows, my network drivers and possibly some of my other software caused this audio problem. I originally could not sample at any setting above 44.1 Khz-16 bit and there was a lot of distortion even then. Now 48 KHz and 24 bit is great with RIAA. Higher speeds and bit levels also worked but I did not write down the details.


On the HP 332A THD + noise at 0.1% (-30dB) was much better, but, still too high for my intended use. The output from the HP 332A, again displayed on the scope, showed a strong approximately 138 KHz signal. As the EMU 0202 measured fine using RIAA, I have to assume that the EMU 0202 filters out high frequency on its input, but it is there on the output. The HP 332A has a very large frequency bandwidth and so its measurement includes this as noise. To be able to use the HP 332A I need to reduce all of the high frequency noise on the output of the EMU 0202. The noise is present at all times, even when no sound is played through the 0202. I used the 332A to null higher frequencies and found that nulling at 138 KHz lowered THD + noise by 3.5 dB. A T-notch filter would look after this, but I need far more. It seems that a high order low pass filter is necessary. One of those high order things I know nothing about.


I'm not very good at this filter thing and would like any comments or suggestions you may have regarding this.


Thanks

Jim
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Old 3rd March 2010, 04:12 PM   #112
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Quote:
Originally Posted by JimT View Post
As some others have reported the EMU 0202, other USB audio devices, generate a lot of noise at their output, far above the advertised level. With the high level of noise I was unable to use this for distortion measurements. I decided to investigate on the off chance I could fix it. I found two individual sources of noise, plus some general background noise.


Please note that the numbers in the following are from memory only and are probably inaccurate.


I connected the 0202 up to an HP 332A distortion meter. played a 1KHz tone using the 0202, and got about 3% THD + noise, worse than my Sansa Clip MP3 player. The output of the HP 332A was connected to an oscilloscope. The oscilloscope showed a large repeating “bump” in the noise about every 250ms. RIAA bounces during its measurement and I assume these noise “bumps” are the cause. Searching the internet lead me to check for latency problems, apparently common with both USB and Firewire devices on Windows. This latency is the delay that Windows can take before responding to a request to service the USB, not a frequency latency as we normally have in audio with crossovers. It seems that if the delay gets too long, the 0202 suffers from Windows transfer "glitches" and noise bumps are the result. The latency was measured with the Thesycon’s DPC Latency Checker (DPC Latency Checker). My laptop had about 500us and was supposedly fine for streaming audio and video. As it was snowing outside and these noise bumps could be caused by USB latency problems, I followed their instructions to reduce this latency and was able to get this down to 6us. Rechecking the output on the scope showed the noise bumps were gone and it seems likely they were caused by these USB latency problems. RIAA now measured quite well at a usable low THD+noise of about -110dB and bouncing during the test was fixed. It seems Windows, my network drivers and possibly some of my other software caused this audio problem. I originally could not sample at any setting above 44.1 Khz-16 bit and there was a lot of distortion even then. Now 48 KHz and 24 bit is great with RIAA. Higher speeds and bit levels also worked but I did not write down the details.


On the HP 332A THD + noise at 0.1% (-30dB) was much better, but, still too high for my intended use. The output from the HP 332A, again displayed on the scope, showed a strong approximately 138 KHz signal. As the EMU 0202 measured fine using RIAA, I have to assume that the EMU 0202 filters out high frequency on its input, but it is there on the output. The HP 332A has a very large frequency bandwidth and so its measurement includes this as noise. To be able to use the HP 332A I need to reduce all of the high frequency noise on the output of the EMU 0202. The noise is present at all times, even when no sound is played through the 0202. I used the 332A to null higher frequencies and found that nulling at 138 KHz lowered THD + noise by 3.5 dB. A T-notch filter would look after this, but I need far more. It seems that a high order low pass filter is necessary. One of those high order things I know nothing about.


I'm not very good at this filter thing and would like any comments or suggestions you may have regarding this.


Thanks

Jim
Hi Jim,

Thanks for this information and the heads-up on USB-connected soundcards. When you said RIAA, did you mean RMAA?

Have you tried any of the other audio analysis software packages to see if they exhibit the same behavior? Lots of them are freeware or shareware with a trial time. Visual Analyzer is one you might try. I'm not a computer geek.

Cheers,
Bob
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Old 3rd March 2010, 05:15 PM   #113
JimT is offline JimT  Canada
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Bob,

Thanks Bob, Yes it was RMAA (RightMark Audio Analyzer ), the latest version 6.2.3. Sorry about that.

I did use other software to play the 1 kHz tone, but only RMAA to check the card. The noise "bump" problem was present with all of them until I "fixed" the USB latency problem. If you don't know how to "disable" your network card and check for the latency you may need the help of a computer geek. Many "kids" will know how to do this. To start them off... you need to right click on "My Computer", select Properties, Device Manager, right click on the network devices, and other things your not using, and disable them. Restart the computer, test using RMAA.

I have not spent much time with analysis software, as I have had issues with sound cards and it seems Windows as well.

Jim
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Old 4th March 2010, 01:25 AM   #114
PB2 is offline PB2  United States
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Bob, I don't think software will make a difference since the input bandwidth, back into the PC, will be band limited by the anti-aliasing filter.

Jim, what exactly are you trying to do. Do you just want to use the 0202 as a signal source? Or do you plan to use it as a source for the external HP analyzer?

What frequency range are you trying to cover using it as a signal source?
If for example you only need to cover 20 kHz with sine waves, then you can place
the low pass filter -3dB cutoff at something like 30 to 50 kHz depending on the
design of the filter. How much attenuation you need above 100 kHz will determine
the order of the filter that you need. I'm not saying that this is the best solution.

What frequency range do you want to cover for analysis of higher order harmonics?
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Last edited by PB2; 4th March 2010 at 01:30 AM.
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Old 4th March 2010, 02:41 AM   #115
PB2 is offline PB2  United States
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Does this help as a reference for the 0202? I don't see the high level of noise that people are talking about within the audio passband based on these measurements:
A Winner

I find that my laptop when powered from the AC adapter will often produce a whine at the output of the internal and external USB audio devices. It is very easy to hear in normal use and for some reason it goes away if I float the AC plug ground. A ground loop obviously but with my laptop and headphones nothing else is grounded, LOL!

I'm interested in the 0202 or 0404 for distortion measurements but have not purchased one yet and so will consider other units also.
I do currently have an M-Audio Transit which is fine for speaker testing.
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Last edited by PB2; 4th March 2010 at 02:44 AM.
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Old 4th March 2010, 05:59 AM   #116
iko is offline iko  Canada
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I didn't even noticed this thread moved along. My 0404 tests pretty much confirm some of your experiences. Was able to get low thd number only when all the stars were properly aligned. Got the best results with the Virtins MI software, after reading the manual. For any sample rate and fft window they recommend a particular frequency for the generated tone. In any case, I had to to use the ASIO drivers, and the 0404 input and output pots had to be set just right to get low thd. The loop cable makes a difference too, I built my own shielded cable. I think the lowest thd I could measure was something like 0.0004, which is, what, 68dB? I've seen some people showed 0.0001, 80dB? Please correct me if my numbers are wrong here.

But I thought there was room for improvement and cracked the 0404 case open today. I had a quick look at the pcb and wrote an initial impression in a wiki (see my sig). I noticed the following:

* the mic/line input uses a 15532 opamp with a data sheet thd of about 0.001

* there are quite a few dc-dc converters getting the right voltages out of the 5V wall wart. Some people claimed that a linear 5V supply made a difference, but my measured thd was the same with the wall wart or with my 5V shunt regulator. Makes sense, because the 5V in gets chopped up and put together again in all the crazy ways to make 12V, 5V, -5V, 1.8V, 3V, etc.

* checked the positive and negative rails of a few opamps with a 60MHz scope with lowest vertical sensitivity of 2mV/div. The positive rail seemed quiet at about 2mV p-p. The negative rail had nasty 6-8mV p-p spikes with a period of about 6.x uS. Incidentally, the dc-dc inverting converter, a TPS6755, works on a switching frequency of 160kHz. Neither the 15532 opamp, nor the other opamp, the 2068 (which is used in great numbers in the 0404) seems to have such great PSRR, so to me, this would be a starting point of improvement, to fix the negative rail voltage. It would be interesting if other people's 0404 or 0202 exhibit the same symptom. Could be that I just got a bad card.

The other obvious target for improvement could be the input and output opamps. There are definitely better opamps that one could use.
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Old 4th March 2010, 10:48 AM   #117
JimT is offline JimT  Canada
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PB2,

I was using the same settings as the person who wrote "A Winner". You are right that the distortion is quite low in the audio band, if I disable my network card and a few other things. My remaining problem is that my distortion meter measures total noise, and as you see on the "A Winner" site, noise increases quite a bit at very high frequencies (over 20 kHz. My meter measures harmonics and noise up to 500kHz. It seems there is a lot of noise in the very very high frequencies and this adds up. If you measure with a device that has less bandwidth you will not notice. If you are using the 0202 to measure itself things will be fine, if you turn off (disable) a few things. Of course if your computer is newer and faster than mine you may not have to jump through these hoops.

Jim
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Old 4th March 2010, 11:05 AM   #118
JimT is offline JimT  Canada
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ikoflexer,

Thanks for your information and work. I'll try to find some time this weekend to look at the negative rail. First I will be trying a low pass filter. As I'm not very good at filters it may take more time than I would like.

Jim
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Old 4th March 2010, 04:57 PM   #119
iko is offline iko  Canada
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Hi Jim,

Just tried something. At the output of the negative rail converter there's a 100uV capacitor from - (output) to +. Yellow little thing SMD, with code 107A on it. Then a tiny resistor with code 0 on it, sort of a CR filter, except that the resistor is just a jumper I think. So I replaced the capacitor with a 470uF OSCON. And replaced the 0 resistor with a small inductor, and then another 470uF OSCON across - and +, basically creating a CLC filter. Indeed this got rid of all the nasty HF spikes on the negative rail.

Running the Virtins MI software and tweaking the input and output volume buttons on the 0404, and the Windows volume button I managed to get 0.0003% (-111.1dB) THD, and 93.3dB SNR. For some reason THD+N is pretty high at 0.0022% (-93.3dB).

In any case, getting rid of those spikes on the negative rail doesn't seem to make so much of a difference, but at least it's less bothersome knowing that they're gone.

By the way I made a mistake in my previous message, I meant to say that before I could measure 0.0004% THD which is not 68dB, but 107.96dB, and 0.0001% THD which is -120dB. Forgot the % :/
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Old 4th March 2010, 08:49 PM   #120
JimT is offline JimT  Canada
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ikoflexer,

Thanks for checking into that. I think my problem may be a bit like the plot in the second post in the following thread:

AK4395/4396 ultrasonic noise spectrum graphic

You can see the large broad peak in noise at about 200kHz. If I can get this high frequency stuff down, I may get close to your level.

Jim
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